( * ... which is sorta Belgium-y German?* )
It's not Belgium-y German at all, just German. It's one of a few terms borrowed from French that exists across the entire language, akin to Croissant; while there are other ways to butter a Croissant so-to-speak ( Hörnchen, or in the Switzerland Gipfeli ), there is no reason to use these synonyms and in fact Croissant is more widespread and better understood. On point, Pommes is universal in German speech everywhere I have been or lived. Pommes Frites I only regularly see on menus, packaging, and advertising.
As for Kartoffelfrites, Frites is not from German, either, so if for the sake of argument the French pommes frites were to be directly transliterated into German, or so to speak Germanized, it would become Bratkartoffeln ... which is an entirely different dish.
This is actually not an exception at all. There are in fact a plethora of feminine-gender German nouns that do not end in -e or -ie. Other endings that are predominantly or entirely feminine are -anz, -ei, -enz, -heit, -in, -ik, -keit, -nis, -schaft, -ion, -sis, -tät, -ung, and -ur. There are very likely as many German feminine words that don't end with -e as there are that do.
"I eat" is not the infinitive -- it's the present simple tense. (er isst could be "he eats", for example, but not "he eat" with the infinitive.)
English makes a distinction here -- between present simple for habitual acts versus present continuous for things that are happening right now -- that German does not make. So when translating from German into English, you have to use the rules of English (habitual? right now?) to determine which tense to use in translation.
Most of Duolingo's sentences have no context you could use for that, and so both translations will usually be possible and should be accepted.